295/45R18 vs 305/40R18
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/45R18 | 305/40R18 | Change |
|---|---|---|---|
| Overall height | 28.5" | 27.6" | -3.0 % |
| Sidewall | 5.23" | 4.80" | -8.1 % |
| Section width | 295 mm | 305 mm | +3.4 % |
| Circumference | 89.4" | 86.7" | -3.0 % |
| Revs per mile | 709 | 731 | +3.0 % |
| Axle height | 14.23" | 13.80" | -10.8 mm |
| Approved rim width | 8.5–11" | 8.5–11" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 295/45R18. Fit 305/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,703 on the road, so it over-counts by 297. At a true 70 mph the first turns 827 times a minute, the second 852.